The present invention relates to an active matrix substrate for a liquid-crystal display, which has a thin-film transistor serving as an active element, and a method of fabricating the active matrix substrate.
In recent years, development in an active-matrix type liquid-crystal display has been remarkable with the object being to make a display flat or plane. The active matrix type is such that a thin-film transistor or a thin-film diode of an active element is incorporated as a switching element in each picture element or pixel. The active matrix type is so characterized as to obtain high picture quality, as compared with a so-called simple matrix type in which transparent electrodes are simply intersected with each other.
As a conventional example, an active matrix substrate will be described here in which a thin-film transistor of poly-crystalline silicon is used. FIGS. 6(a) and 6(b) show the conventional example which is disclosed in "SID '88 Digest", H. Ohshima et al., Lecture No. 21.4, pp 408-411. FIG. 6(a) is a top plan view showing one of a plurality of picture elements, while FIG. 6(b) is an enlarged cross-sectional view taken along the line VI(b)--VI(b) in FIG. 6(a).
The conventional active matrix substrate comprises a glass substrate 1 and a pair of source and drain regions 3 and 2 on the glass substrate 1. The source and drain regions 3 and 2 are composed of poly-crystalline silicon in which phosphorus is doped at high concentration. An active layer 4 composed of an undoped poly-crystalline silicon thin film is provided on the source and drain regions 3 and 2 in contact therewith. The active layer 4 is covered with a gate insulating film 5 made of silicon dioxide. On the gate insulating film 5 there is provided a gate electrode and gate line 6 made of chromium. The gate electrode 6 is covered with an interlayer insulating film 7 made of silicon dioxide. The interlayer insulating film 7 is partly covered with a picture-element electrode 8 which is composed of a transparent conductive element made of indium tin oxide. A drain signal line 9 is provided which is made of aluminum.
An example of a method of fabricating the active matrix substrate is illustrated in FIGS. 7(a) through 7(d) and will next be described in due order.
(1) A poly-crystalline silicon containing a high concentration of phosphorus is formed on a glass substrate by a low-pressure CVD (chemical vapor deposition) process which uses phosphine (PH.sub.3) and silane (SiH.sub.4) as raw-material gas. The poly-crystalline silicon is patterned to form the source region 3 and the drain region 2. Subsequently, silane is decomposed similarly by the low-pressure CVD process to form a poly-crystalline silicon thin film on the source and drain regions 3 and 2. The poly-crystalline silicon thin film is patterned in the form of an island to form the active layer 4. PA1 (2) The gate insulating film 5 consisting of a silicon dioxide film is formed on the active layer 4 by a CVD process which uses silane and oxygen as raw-material gas. Chromium is formed on the gate insulating film 5 by means of sputtering and is subsequently patterned to form the gate electrode 6. PA1 (3) The interlayer insulating film 7 consisting of silicon dioxide is formed on the gate electrode 6 again by means of the CVD process. A pair of contact holes to the respective source and drain regions 3 and 2 are formed in the silicon dioxide films 5 and 7. PA1 (4) Lastly, the drain line 9 made of aluminum and the picture-element electrode 8 made of indium tin oxide are formed into their respective films by means of a sputtering process or a vacuum deposition process and, subsequently, are patterned into their respective desired configurations. Thus, the fabricating method is completed.
The above-described construction and fabricating method have various problems caused by the use of the indium tin oxide. First, inferiority in etching of the indium tin oxide can be given. That is, the indium tin oxide has the following problems. Although superior in nature as a transparent conductive film, the indium tin oxide involves difficulties in dry etching, is not so good in processing accuracy at wet etching, and so on. For this reason, it is necessary to take a sufficient space more than 5 .mu.m between the line and the individual transparent electrode made of indium tin oxide. This space becomes one of causes which reduce the area of the transmitting section of a light. Secondly, there is such a problem that etchant for the wet etching etches also aluminum. Accordingly, in the case where the indium tin oxide and the aluminum line are on the same or identical plane as in this example, a very high etching technique is required. Thirdly, there is such a problem that the indium tin oxide is apt to be damaged by hydrogen plasma. Although it is necessary to decrease the trap density in grain boundary as a necessity peculiar to the poly-crystalline silicon, the hydrogen plasma treatment is a technique which is often used. In this construction, however, since the indium tin oxide forms the uppermost layer, it is difficult to improve the transistor characteristic by use of the hydrogen plasma treatment at the last step. As another problem, the involvement of a number of the fabricating steps can be given. In this example, six (6) masks are used. The large number of the steps causes reduction of the yield.